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 Post subject: mistake in article?Posted: Fri Feb 13, 2009 7:49 am

Joined: Mon Jul 16, 2007 1:03 am
Posts: 22
hello

I have read in the articles that the noise that comes up from a component may be 'covered' by the ambient noise of the room or the noise from another component

is this true?

I mean, if a component emits 15db and another emits 20db, the noise won't be 15+20=35db? it may be just 20db (with the other 15db being "covered") ?

ofcourse, it may depend on the wave length of the noise and on the way the ear perceives the noises

I would like more facts or even opinions on this

thanks

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 Post subject: Posted: Fri Feb 13, 2009 7:59 am

Joined: Sun May 21, 2006 9:09 am
Posts: 2269
Location: Northern California.
so if you run four 30db fans it should be as loud as a jet engine?

not much to discuss, but lots to read on the subject if you look...

Quote:
Adding Sounds or Noises together on the Decibel Scale

In real life, several sources of sounds often occur at the same time. One may be interested to know what results when one sound is combined with another, i.e. the addition of sounds.

Adding 60 apples to 60 apples results in 120 apples. But this is not the case with sounds when they are expressed in decibels. In fact, adding 60 decibels to 60 decibels gives 63 decibels.

Quote:

Sound pressure levels in decibels (dB) or A-weighted decibels [dB(A)] are based on a logarithmic scale (see Appendix A). They cannot be added or subtracted in the usual arithmetical way. If one machine emits a sound level of 90 dB, and a second identical machine is placed beside the first, the combined sound level is 93 dB, not 180 dB.

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 Post subject: Posted: Fri Feb 13, 2009 10:07 am

Joined: Wed Aug 23, 2006 7:09 pm
Posts: 536
Assuming the wave's are in phase then a doubling of audio power (translates as volume) is a 3 dB increase.

However what a person hears goes beyond the numbers. Sound waves of almost equal content and out of phase can perceptually reduce the effective level by canceling out the sound waves from another source. The frequency of the sounds also plays a role.

This is the principle behind using "white" or "pink noise" to mask out other sounds. This is often used in movies where someone runs a shower and then holds their conversation in the bathroom where the noise from the shower masks most of the voices.

_________________
Obsolesence is just a lack of imagination!

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 Post subject: Re: mistake in article?Posted: Fri Feb 13, 2009 10:26 pm

Joined: Tue Feb 13, 2007 7:28 pm
Posts: 1406
Location: USA
color wrote:
I mean, if a component emits 15db and another emits 20db, the noise won't be 15+20=35db? it may be just 20db (with the other 15db being "covered") ?

15dB + 20dB is not 35dB. 15dB + 20dB is 21dB. Why is that? Because dB is a logarithmic scale of relative values. 10dB = 10X, 20dB = 100X, 30dB = 1000X, and so on. To make matters more confusing, dB in terms of sound is a measure of intensity, not perceived loudness. 10X intensity only translates to 2X perceived loudness. Also, as NyteOwl notes, the sounds have to be perfectly in phase to be perfectly additive. How in phase the sounds are is of course completely dependent on listener position. This means that in real life, two noise sources are likely to always have a composite intensity at the listener position that is less than the combined value of each individual sound. Finally, human hearing is a very complicated system and you actually have the ability to "tune in" on sounds that are below the ambient noise or covered by the noise of other sounds. This tuning in process can happen subconsciously. So, to sum it all up: Yes, louder sounds and or ambient conditions can cover up some other sound. However, that even if the new sound is covered up for the purpose of measurement with s sound meter, that doesn't mean you won't hear it!

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